The remediation of spills that contaminate an underground zone can require drilling one or more wellbores into the contaminated zone. The wellbores provide a conduit for contaminated fluids to be withdrawn from the formation to the surface for treatment or a conduit for treatment fluids from the surface to be injected into the underground zone. In either case, significant fluid flow within the zone to or from the well must be accomplished, e.g., the zone must be sufficiently porous and permeable to fluid flow.
Although some underground formations have acceptable fluid permeability and porosity, i.e., allow fluid movement within the formation, other formations present significant resistance or barriers to fluid movement. These less permeable formations may require added process steps and measures to allow fluid to be withdrawn or injected, e.g., multiple wells drilled within a formation (i.e., each well having only a limited radial zone of influence within the formation from the wellbore), larger diameter wellbores (to increase cross-sectional flow area at the wellbore face), and high pressure pumps (to overcome a larger resistance to fluid flow).
If these added measures are not sufficient, formation altering methods, such as acidification and fracturing, can be used to increase permeability or otherwise provide improved fluid paths within the formation. Formation altering methods tend to initiate alterations at the wellbore and propagate the alteration outward from the wellbore into the formation.
However, formation altering methods also present major risks. The methods may adversely affect subsequent remediation steps, e.g., allow contaminated fluids to move out of the contaminated zone prior to treatment. The methods may also adversely impact post-remediation uses of the zone, e.g., rupturing a shale barrier which would have tended to contain future spills.
The risks of formation altering are magnified when the contaminated zone is a relatively thin layer located close to the surface, e.g., contaminated fluids in a vadose zone above a potable groundwater table. The added risks include a risk to damage to surface equipment, a risk of unwanted ejection of contaminated fluids at the surface, a risk of damage to or contamination of shallow ground water resources, and a risk of damage to nearby utility conduits buried at shallow depths.
These formation altering risks are still further magnified if these formation altering methods are applied from highly deviated wells, such as horizontal wells, within the vadose zone. The surface rupture risk and/or the risk of propagation out of a thin vadose layer may be especially difficult to avoid over the extended length of a horizontal wellbore.